1. Field of the Invention
The present invention relates to an optical disc reproducing apparatus.
2. Description of Related Art
A technique for converting analog audio data into digital audio data (hereinafter to be simply referred to as “audio data”) has been widely known. Compression techniques which can reduce an amount of audio data are also known. A plurality of standards for generating compressed audio data are known. For example, typical standards include MP3, WMA and AAC.
Along with a progress of information technology, techniques for recording the compressed audio data on an optical information recording medium such as CD-R and DVD-R are widespread. The compressed audio data recorded on the optical information recording medium is converted into analog audio data by an optical disc reproducing apparatus (for example, a CD drive) and then the audio data is outputted from an output unit such as a speaker.
The optical disc reproducing apparatus for the compressed audio data is provided with a buffer memory. The optical disc reproducing apparatus executes processes such as a decoding process on read data read out from the optical information recording medium and held in the buffer memory, and then suppresses such fault that audio data which is being reproduced is suspended.
When data is sequentially transferred from the optical disc reproducing apparatus to a host computer, the data after a decoding operation by a decoding circuit is stored in a buffer if a transfer rate is smaller than a read rate from the optical disc. When the read data is fully written into the buffer memory, a read operation from the optical disc must be brought to a halt until the buffer memory has a room for storage. In the meantime, power may be consumed due to rotation of the optical disc. Thus, a technique of varying rotation speed of the optical disc depending on an amount of data held in the buffer memory is known in Japanese Patent Application Publication (JP-P2003-242708A: conventional example 1).
FIG. 1 is a block diagram showing an information recording disc reproducing apparatus described in the conventional example 1. In the information recording disc reproducing apparatus described in conventional example 1, rotation of the information recording disc 103 is controlled by a DVD-ROM drive 102 and a signal read from the information recording disc 103 is decoded by the DVD-ROM drive 102 and transferred to a host computer 101. The DVD-ROM drive 102 includes a microprocessor 104, a ROM 105, a buffer 106, a reproduction speed control section 107, an information recording disc reading apparatus 108, a decoder 109 and a RAM 110.
According to a program previously stored in the ROM 105 and an instruction issued from the host computer 101, the microprocessor 104 controls other blocks in the DVD-ROM drive 102. Data read out from the information recording disc 103 by the information recording disc reading apparatus 108 is supplied to a decoder 109 and decoded by the decoder 109. The decoded data is sequentially stored in the buffer 106 and, after temporary storage, is transferred to the host computer 101. Depending on a transfer type such as transfer method or transfer mode for a transfer section adapted to transfer data to the host computer 101, a line speed at a reproduction position of the information recording disc 103 is controlled to be highest line speed determined based on a transfer rate of each transfer type or less. By controlling the line speed at the reproduction position of the information recording disc 103 so that a data transfer rate to the host computer 101 does not exceed a data rate at which the decoded data is stored in the buffer 106, a high-speed rotation of the information recording disc 103 is suppressed. When the amount of data stored in the buffer 106 is a predetermined value or more, the buffer 106 is prevented from being fully filled by reducing a rotation speed of the information recording disc 103.
The conventional example 1 describes a technique of an information recording disc reproducing apparatus which varies the rotation speed of the information recording disc 103 depending on a transfer section. The decoder 109 of the information recording disc reproducing apparatus performs a CD decoding process and a CD-ROM decoding process to the CD-ROM disc. Here, when the data recorded on the information recording disc 103 is compressed audio data, the decoder 109 decodes the compressed audio data in addition to the CD decoding process and the CD-ROM decoding process.
When multiple compressed audio data having different data compressibilities are recorded on the information recording disc 103, a decoding process corresponding to the data compressibility is performed. For this reason, the data rate at which the decoded data is stored in the buffer 106 varies depending on the decoding process. That is to say, when the technique described in conventional example 1 is applied to the optical disc reproducing apparatus capable of reproducing the compressed audio data, it may be difficult to determine a data rate in the decoding process. In addition, it may be difficult to determine a line speed in read of the optical disc.
Furthermore, there is a variable bit rate method in which the data compressibility of a part of the compressed audio data is not fixed but variable. When the technique described in conventional example 1 is applied to the optical disc reproducing apparatus capable of reproducing the compressed audio data, it is difficult to cope with variance of a decoded data rate.
Unlike an apparatus in which the line speed of the optical disc can be calculated by an external transfer section, the line speed of the optical disc which stores the compressed audio data cannot be previously known. Thus, conventional systems cannot rotate at a low speed and control the optical disc which stores the compressed audio data. There is a conventional method of monitoring the number of stages of FIFO and performing a phase comparison of data write/read into/from a buffer memory. However, when data rates of data write/read into/from the buffer memory are different or the data rate is extremely low, there is no technique for coping with such a state.